The following discussion deals with starter tubes and sealing arrangements for optical fiber pre-forms, but it is to be understood that principles of the present invention are applicable to other, different applications involving, generally, chemical delivery systems wherein the chemicals are in gaseous or vapor form.
Optical fiber of the type used to carry optical signals is fabricated typically by heating and drawing a portion of an optical pre-form comprising a refractive core surrounded by a protective glass cladding. Presently, there are several known processes for fabricating pre-forms. The modified chemical vapor disposition (MCVD) process, which is described in U.S. Pat. No. 4,217,027 issued in the names of J. B. MacChensey et al. on Aug. 12, 1980 and assigned to Bell Laboratories, Inc. has been found to be one of the most useful because the process enables large scale production of pre-forms which yield very low loss optical fiber.
During the fabrication of pre-forms by the MCVD process, reactant-containing gases, such as SiCL.sub.4 are passed through a rotating substrate tube which is made of silica glass. A torch heats the tube from the outside as the precursor gases are passed therethrough, causing deposition of submicron-sized glass particles on the inside surface of the tube. The torch is moved along the longitudinal axis of the tube in a plurality of passes to build up layer upon layer of glass to provide a pre-form tube. Once a sufficient number of layers have been deposited, the pre-form tube is then heated to cause it to be collapsed to yield a pre-form or pre-form rod as it is often called. The delivery system of the reactant gases to the starter tube interior is generally through a fixed metallic hollow tube connected to the source or sources of the gases. It is necessary that the space between the exterior surface of the delivery tube and the interior surface of the glass starter tube be sealed so that the critical gases do not leak out of the starter tube.
In the current method of manufacture, the apparatus which ensures sealed delivery of the deposition chemicals in the gases is a combination of a rotary union element, a structure for holding and sealing the starter tube, and a secondary face seal assembly for routing of purge gases through the structure. This is a complex apparatus that requires frequent maintenance. Existing systems also have the disadvantage of having inherently larger cavities for the accumulation of dead zones of flow, and a tendency to create particle contamination from the rotary union and face seal system. Inasmuch as the chemical delivery system is stationary, the current means of achieving delivery is via the rotary union, featuring a transition of the chemicals from a stationary pipe to a rotary pipe or to the inside of a supply coupling. The chemicals being delivered are at a pressure greater than atmospheric, and the face seal properties are the only restriction to the release of the chemicals to the atmosphere. The rotary union and secondary face seals generate a large quantity of particles from wear, and contribute to the contamination of the coupling. As the seals wear down, excessive leakage occurs both to the atmosphere and into the product (starter tube), resulting in lost product and requiring system maintenance. The complexity of the components involved requires skilled maintenance being performed using requalification through test of the system. Both material and labor costs are, consequently, high.
Present day techniques and apparatus provide a buffer zone of gas/liquid between the chemical being delivered into the atmosphere. The buffer zone provides for a controlled environment, which may be regulated to pressures higher or lower than the surrounding chemical being delivered and higher/lower than the atmosphere. Selection of this buffer zone pressure and constituent gas/liquid in this zone controls the leakage directions and the leakage constituents across the seals. In a typical buffered sealing system, the buffered volume is achieved most simply by having a single internal seal and a single external seal connected via a housing which is fixed relative to the rotating member to be sealed. The introduction passage and the removal passage are then connected through the housing to the buffer volume.
In the aforementioned Mueller patent application Ser. No. 09/383,716, there is shown a sealing system that eliminates many drawbacks characteristic of prior art delivery systems, as enumerated in that application, such as, for example, the rotary union, by internally sealing the starter tube by means of a self tightening seal and mounting arrangement therefor. The basis of such the arrangement of that application requires a constant rotational capability of the mounting hub for the self tightening feature, which conflicts with the need for fixed connections to the buffer zone of the introduction and removal lines. Inasmuch as supply lines cannot be connected directly to the components of the Mueller arrangement, the advantages to be gained through the use of disposable, self-tightening seals would appear to be unrealizable.